Chrome plating is a process that has been utilized for many years to provide a decorative chrome finish on a metal surface. One particular application for chrome plating has been in the automotive industry where the appearance of metal finishes is a significant concern. While chrome plating of metal surfaces has been utilized for many years, it is expensive and has corrosion issues.
Over time, alternatives to these metal chrome plating processes have been developed. Some of these developments were made in response to concerns over the use and disposal of hexavalent chromium and heavy metals typically used in the plating process. Additionally, regulations surrounding the use of hexavalent chromium have become more restrictive, causing efforts aimed at developing alternative technologies to correspondingly increase. A number of commercial chemistries using trivalent chromium have been offered as replacements for hexavalent chromium in the plating process to address these concerns. Processes using these chemistries, however, provide decreased corrosion protection and are significantly more expensive. Further, while trivalent chromium does not possess the same toxicity as hexavalent chromium, it still poses environmental challenges. These developments thus have not produced commercially viable alternatives to metal chrome plating processes.
It is known that the use of metal as a base material provides a part that is heavy and expensive. Thus, alternative processes were developed that involved substituting plastic for metal as the base material such that a metal surface is plated over the plastic base material. One such process involves etching the plastic surface of the article and then depositing an active catalyst on the etched surface. Thereafter, a layer of metal is plated onto the surface via an electroless process. In accordance with this process, copper layers are then electrolytically deposited thereon for leveling and conductivity purposes. Additionally, nickel layers are deposited on top of the copper layers for leveling and corrosion protection. Lastly, a final chromium layer is added to the surface for color and reflectivity. This process yields an article that is durable, corrosion resistant, and highly reflective making it suitable for exterior and interior automotive applications.
In accordance with this process, to provide the increased durability over prior processes, the surface of the plastic substrate is aggressively roughened to provide adequate adhesion between the plastic surface and the deposited metal. The surface is typically aggressively roughened to achieve a root mean square roughness in excess of a few microns. This aggressive roughening provides mechanical interlocks that are capable of allowing adherence of a metal surface thereto under a wide range of temperatures and mechanical stresses. Additionally, to ensure durability of the metal finish created with these prior processes, the metal layers applied to the substrate have a relatively large thickness, which is typically in the range of 40 microns or more. A primary reason for the relatively large thickness with these prior metal finishes results from the need for leveling of the metal layers. Application of a thick layer of metal to the aggressively roughened surface provides sufficient adherence and durability to withstand high temperatures and stresses, such as those encountered in the automotive industry and other similar applications. While this process is commercially successful, it would be desirable to develop an improved process that has decreased cost and/or yields other benefits.
Another alternative to metal chrome plating that has been developed and utilized is vacuum metalizing. Vacuum metalizing has the benefit of providing a more reflective metal surface than prior chrome plating processes. However, vacuum metalizing is extremely costly, is a time consuming process, and has size and part restrictions. For example, vacuum metalizing is not suitable for parts having complex geometry or being of a large size. Additionally, parts subjected to vacuum metalizing have limited durability. Vacuum metalizing therefore has only limited applications and poses some substantial disadvantages.
Other technologies for plating plastic surfaces have also been commercialized. In accordance with one process, a base coat is deposited on a substrate material and then a spray-on silver metal is deposited over the base coat to provide a reflective surface. Next, a tinted clear protective top coat is applied over the silver metal deposit. With this process, the reflective silver layer is attained by spraying two water-based formulations, a silver salt solution and a reducing solution, at the same time through a specialized dual-nozzle spray gun. The two solutions mix and react directly in front of the gun's nozzles, yielding a fine mist of silver metal which deposits on the base coat surface. While this process provides a finish with a pleasant appearance, it does not have sufficient durability for automotive applications, due in part to its inability to withstand known thermal shock tests. Moreover, this technology has volume restrictions and can only be utilized with surfaces having relatively simple configurations.
Still another alternative to metal chrome plating processes is tin-cobalt alloy electroplating, which has been utilized in applications in the furniture industry. While tin-cobalt alloy electroplating provides a quality metal appearance, its durability is limited as well as its corrosion resistance. The durability and corrosion resistance may be acceptable for the furniture industry, however, it cannot withstand the stringent performance requirements of automotive applications and other applications requiring a durable metal finish such as consumer electronics.
It would therefore be desirable for economic and environmental reasons to develop an alternative technique or process capable of providing durable reflective decorative finishes that can be utilized in industries where the surfaces would be subjected to environmental and mechanical stresses and that can be produced in mass volume without restrictions on part size or geometry and yields cost savings.